Combustion of biomass has been used by humankind to generate heat and light for about 500 millennia. Biomass was the world's predominant energy source until fossil fuels took over in the industrial world, during the industrial revolution. As a renewable energy source, biomass can now re-assume greater energy loads while serving a number of other economic, environmental, and social purposes. Biomass can include, for example, wood, wood waste, agricultural waste, energy crops, municipal solid waste, sewage sludge and cellulosic type industrial waste.
Biomass is made by photoreactions such as EQU 6CO.sub.2 +5H.sub.2 O sunlight C.sub.6 H.sub.10 O.sub.5 +6O.sub.2
leading to cellulosic type compounds such as C.sub.6 H.sub.10 O.sub.5. Biomass fuel is greenhouse neutral since when burned it simply releases carbon dioxide and water that it took out of the atmosphere when it grew.
The conversion of heat to mechanical work can be accomplished via hot air engines, steam engines or internal combustion engines (ICE). Hot air and steam can be made by the direct combustion of biomass. However, for use with reciprocating or turbine ICEs, which have been dominant in the 20.sup.th century, biomass is best converted to liquid or gaseous fuels.
Gasification is a thermal process of changing a solid fuel such as coal, biomass or municipal solid waste into combustible gas and oil vapors. Four conventional biomass gasifier types that have evolved over many years are the fixed-bed updraft gasifier, the fixed bed downdraft gasifier, the moving bed gasifier, and the fluidized bed gasifier. Each type has advantages which are dependent upon the operating conditions, the output power required, and various other factors.
These conventional gasifiers typically create the heat for gasification by burning fuel in the gasification chamber itself. This can involve injecting air with its nitrogen into the gasification chamber, which dilutes the output gas with inert nitrogen as well as the products of combustion, including CO.sub.2 and H.sub.2 O. The fuel being burned to generate the heat for gasification is typically burned without sufficient oxygen, thus creating the fuel gas carbon monoxide.
A new class of gasifiers, referred to as indirectly heated gasifiers (IHGs), can generate at least a portion of the heat of gasification by combustion carried out in a separate chamber from the gasifier reactor. Accordingly,these IHGs reduce dilution of the output gas with nitrogen, carbon dioxide and water vapor. For example, one such gasifier heats sand in a separate combustion chamber and then transfers the hot sand into the gasification chamber to provide the heat for gasification. However, this process of heating and transferring sand is complicated and applies mainly to large gasification systems.
Depending upon the gasification agent and the gasifier, typical biomass gasifiers produce combustible CO, H.sub.2, CH.sub.4, and other light hydrocarbons, diluted with non-combustibles N.sub.2, CO.sub.2 and H.sub.2 O vapor. The heat of combustion of the product gas is determined by the biomass feedstock and gasification agent used, as well as by the operational conditions, such as pressure, temperature, residence time and heat loss or external heat input.
The types of gases produced by biomass gasification can be divided into three categories according to their heat value (HV). Low heating value (LHV) gas (.about.6 kJ/liter) is produced by traditional gasifiers when air is used as the gasifier agent. The gas is used on site since storage and/or transportation of LHV gas are not economically favorable. Medium heating value (MHV) gas (.about.13 kJ/liter) can be produced with traditional gasifiers when oxygen is used as the gasifier agent since dilution by nitrogen is avoided. Medium HV gas can be used as fuel for internal combustion engines and gas turbines. In addition, medium HV gas can also be produced with IHGs in which the combustion chamber and the gasification chamber are separated. In IHGs the gasification process takes place without external oxygen (or nitrogen) and the output gas consists mostly of carbon monoxide with varying concentrations of the fuels hydrogen, methane, ethylene, ethane and other hydrocarbons, as well as some non-fuels such as carbon dioxide and water vapor. Medium HV gas can be used for the production of synthetic fuels, such as H.sub.2, gasoline, methanol, synthetic natural gas, etc. High HV gas (.about.37 kJ/liter) is usually produced from medium HV gases. These gases can be used as substitutes for natural gas which usually has a heating value of about 50 kJ/liter.
Gas turbines (GT) have shown promise as an efficient means of transforming heat into mechanical work and are now serving as major components of large new electricity generation systems using natural gas. For example, a low cost solid fuel (SF) cogasifier fed by low cost local feedstocks can be coupled with smaller GT systems adapted for medium HV gas to produce electricity in, for example, remote regions where availability of electricity is limited. The cogasification of biomass with other domestic fuels can provide a long term strategy for effective utilization of biomass. For example, the blending of oxygenated fuels such as biomass with carbonaceous fuels such as coals, coke, and chars in a small cogeneration system can have technological, economic, and environmental advantages. In addition, interest in distributed electricity generation is creating a need for low cost gasifiers.
Even with a low cost gasifier, a gasifier-microturbinegenerator (GMTG) might have difficulty these days competing economically with simpler natural gas microturbine generating systems in many locations because of the current low price of natural gas in the United States. However GMTGs could be economically competitive if they could also serve secondary and tertiary added value functions. The standby production of liquid fuels and chemicals from biomass is an example of a useful secondary function that could help in amortizing the capital cost of a new gasifier. Another societal function that could be served in the longer term by an IHG biomass gasifier is the gasification or liquefaction of the organic matter in metal ladened biomass with the concentration of toxic metals is the char ash. Examples of this need arise in the disposal of plants used in phytoremediation of toxic sites or the disposal of copper chromium arsenate (CCA) treated wood that has exceeded its lifespan. Additional functions, such as the use of the microturbine exhaust output for generating process steam or heat for drying crops or biomass fuel, would further enhance the economic value of the GMTG system.